Double Neutron Star Formation Research Articles

Electron-capture supernovae in NS + He star systems and the double neutron star systems

ABSTRACT Electron-capture-supernovae (EC-SNe) provide an alternative channel for producing neutron stars (NSs). They play an important role in the formation of double NS (DNS) systems and the chemical evolution of galaxies, and contribute to the NS mass distribution in observations. It is generally believed that EC-SNe originate from e-captures on $\rm ^{24}Mg$ and $\rm ^{20}Ne$ in the massive degenerate oxygen–neon (ONe) cores with masses close to the Chandrasekhar limit (MCh). However, the origin of EC-SNe is still uncertain. In this paper, we systematically studied the EC-SNe in NS + He star systems by considering the explosive oxygen burning that may occur in the near-MCh ONe core. We provided the initial parameter spaces for producing EC-SNe in the initial orbital period − initial He star mass (log$P_{\rm orb}^{\rm i}-M_{\rm He}^{\rm i}$) diagram, and found that both $M_{\rm He}^{\rm i}$ and minimum $P_{\rm orb}^{\rm i}$ for EC-SNe increase with metallicity. Then, by considering NS kicks added to the newborn NS, we investigated the properties of the formed DNS systems after the He star companions collapse into NSs, such as the orbital periods, eccentricities, and spin periods of recycle pulsars (Pspin), etc. The results show that most of the observed DNS systems can be produced by NS kicks of $\lesssim$50 km s−1. In addition, we found that NSs could accrete more material if the residual H envelope on the He star companions is considered, which can form the mildly recycled pulsars (Pspin ∼ 20 ms) in DNS systems.

Double neutron star formation via consecutive type II supernova explosions

ABSTRACT Since the discovery of the first double neutron star (DNS) system, the number of these exotic binaries has reached 15. Here we investigate a channel of DNS formation in binary systems with components above the mass limit of Type II supernova explosion (SN II), that is, 8M⊙. We apply a spherically symmetric homologous envelope expansion model to account for mass-loss, and follow the dynamical evolution of the system numerically with a high-precision integrator. The first SN occurs in a binary system whose orbital parameters are pre-defined, then, the homologous expansion model is applied again in the newly formed system. Analysing 1 658 880 models we find that DNS formation via subsequent SN II explosions requires a fine-tuning of the initial parameters. Our model can explain DNS systems with a separation greater than 2.95 au. The eccentricity of the DNS systems spans a wide range thanks to the orbital circularization effect due to the second SN II explosion. The eccentricity of the DNS is sensitive to the initial eccentricity of the binary progenitor and the orbital position of the system preceding the second explosion. In agreement with the majority of the observations of DNS systems, we find the system centre-of-mass velocities to be less than 60 km s−1. Neutron stars that become unbound in either explosion gain a peculiar velocity in the range of 0.02 − 240 km s−1. In our model, the formation of tight DNS systems requires a post-explosion orbit-shrinking mechanism, possibly driven by the ejected envelopes.

The observed velocity distribution of young pulsars – II. Analysis of complete PSRπ

ABSTRACT Understanding the natal kicks, or birth velocities, of neutron stars is essential for understanding the evolution of massive binaries and double neutron star formation. We use maximum likelihood methods as published in Verbunt et al. to analyse a new large data set of parallaxes and proper motions measured by Deller et al. This sample is roughly three times larger than number of measurements available before. For both the complete sample and its younger part (spin-down ages τ < 3 Myr), we find that a bimodal Maxwellian distribution describes the measured parallaxes and proper motions better than a single Maxwellian with probability of 99.3 and 95.0 per cent, respectively. The bimodal Maxwellian distribution has three parameters: fraction of low-velocity pulsars and distribution parameters σ1 and σ2 for low- and high-velocity modes. For a complete sample, these parameters are as follows: $42_{-15}^{+17}$ per cent, $\sigma _1=128_{-18}^{+22}$ km s−1, and σ2 = 298 ± 28 km s−1. For younger pulsars, which are assumed to represent the natal kick, these parameters are as follows: $20_{-10}^{+11}$ per cent, $\sigma _1=56_{-15}^{+25}$ km s−1, and σ2 = 336 ± 45 km s−1. In the young population, 5 ± 3 per cent of pulsars have velocities less than 60 km s−1. We perform multiple Monte Carlo tests for the method taking into account realistic observational selection. We find that the method reliably estimates all parameters of the natal kick distribution. Results of the velocity analysis are weakly sensitive to the exact values of scale lengths of the Galactic pulsar distribution.

LISA and the Existence of a Fast-merging Double Neutron Star Formation Channel

Abstract Using a Milky Way (MW) double neutron star (DNS) merger rate of 210 Myr−1, as derived by the Laser Interferometer Gravitational-Wave Observatory (LIGO), we demonstrate that the Laser Interferometer Space Antenna (LISA) will detect on average 240 (330) DNSs within the MW for a 4 yr (8 yr) mission with a signal-to-noise ratio greater than 7. Even adopting a more pessimistic rate of 42 Myr−1, as derived by the population of Galactic DNSs, we find a significant detection of 46 (65) MW DNSs. These DNSs can be leveraged to constrain formation scenarios. In particular, without prior information on a particular system’s position and orbital period, traditional NS-discovery methods using radio telescopes alone are insensitive to DNSs with P orb ≲ 1 hr (merger times ≲10 Myr). If a fast-merging channel exists that forms DNSs at these short orbital periods, LISA affords, perhaps, the best opportunity to observationally identify and characterize these systems; we show that toy models for possible formation scenarios leave imprints on DNS orbital eccentricities, which may be measured by LISA for values as small as ∼10−2.

Common envelope episodes that lead to double neutron star formation

AbstractClose double neutron stars (DNSs) have been observed as Galactic radio pulsars, while their mergers have been detected as gamma-ray bursts and gravitational wave sources. They are believed to have experienced at least one common envelope episode (CEE) during their evolution prior to DNS formation. In the last decades, there have been numerous efforts to understand the details of the common envelope (CE) phase, but its computational modelling remains challenging. We present and discuss the properties of the donor and the binary at the onset of the Roche lobe overflow (RLOF) leading to these CEEs as predicted by rapid binary population synthesis models. These properties can be used as initial conditions for detailed simulations of the CE phase. There are three distinctive populations, classified by the evolutionary stage of the donor at the moment of the onset of the RLOF: giant donors with fully convective envelopes, cool donors with partially convective envelopes, and hot donors with radiative envelopes. We also estimate that, for standard assumptions, tides would not circularise a large fraction of these systems by the onset of RLOF. This makes the study and understanding of eccentric mass-transferring systems relevant for DNS populations.

The Rate of iPTF 14gqr like Ultra-stripped Supernovae and Binary Evolution Leading to Double Neutron Star Formation

Abstract Double neutron star (DNS) systems are produced from massive binaries. A supernova (SN) explosion of an extremely stripped star is expected to occur at the final stage of DNS formation. This type of SN is called an ultra-stripped SN (USSN). Recent research revealed that a type Ic SN, iPTF 14gqr (SN 2014ft), has low ejecta mass (≈0.2 M ⊙) and its progenitor has a helium envelope with mass ∼0.01 M ⊙. This SN is interpreted as a USSN, and thus this is the first discovery of a USSN. Furthermore, the observation of iPTF 14gqr provides us with some information about its formation history. Here, we perform rapid population synthesis calculations so as to estimate the detection rate of iPTF 14gqr like USSNe with optical transient surveys: the intermediate Palomar Transient Factory (iPTF), the Zwicky Transient Facility (ZTF), and the Large Synoptic Survey Telescope (LSST). We find that iPTF, ZTF, and LSST can observe iPTF 14gqr like USSNe at rates of 0.3, 10, and 1 yr−1, respectively. The iPTF can detect 1 iPTF 14gqr like USSN during its four year observation. We also investigate effects of mass-loss efficiency during Roche-lobe overflow on formation channels.

Double neutron star formation: merger times, systemic velocities, and travel distances

The formation and evolution of double neutron stars (DNS) have traditionally been studied using binary population synthesis. In this work, we take an alternative approach by focusing only on the second supernova (SN) forming the DNS and the subsequent orbital decay and merger due to gravitational wave radiation. Using analytic and numerical methods, we explore how different NS natal kick velocity distributions, pre-SN orbital separations, and progenitor He-star masses affect the post-SN orbital periods, eccentricities, merger times, systemic velocities, and distances traveled by the system before merging. Comparison with the set of 17 known DNSs in the Milky Way shows that DNSs have pre-SN orbital separations ranging between 1 and 44 $R_{\odot}$. Those DNSs with pre-SN separations $\sim$1 $R_{\odot}$ have merger time distributions that peak $\sim$10-100 Myr after formation, regardless of the kick velocity received by the NS. These DNSs are typically formed with systemic velocities $\sim$10$^2$ km s$^{-1}$ and may travel $\sim$1-10 kpc before merging. Depending on progenitor mass of the second-born NS, the short merger time can account for the $r$-process enrichment observed in compact stellar systems such as ultra-faint dwarf galaxies. For Milky Way-mass galaxies only DNSs with the tightest pre-SN orbits and large kick velocities ($\gtrsim$10$^2$ km s$^{-1}$) can escape. However, those DNSs that do escape may travel as far as $\sim$Mpc before merging, which as previous studies have pointed out has implications for identifying the host galaxies to short gamma ray bursts and gravitational wave events.

The masses of 18 pairs of double neutron stars and implications for their origin

AbstractFor the observed 18 pairs of double neutron star (DNS) systems, we find that DNS mass distribution is very narrow and its mean value (about 1.34 solar mass) is less than the mean of all measured pulsars of about 1.4 solar mass. To interpret the special DNS mass characteristics, we analyze the DNS formation process, via the phases of HMXBs, by investigating the evolution of massive binary stars. Moreover, in DNSs, two classes of NSs are taken into account, formed by supernova (SN) and electron capture (EC), respectively, and generally the NS mass by SN is bigger than that by EC. Quantitatively, with various initial conditions of binary stars, the observed special DNS distribution can be satisfactorily explained.

The progenitors of compact-object binaries: impact of metallicity, common envelope and natal kicks

Six gravitational wave events have been reported by the LIGO-Virgo collaboration (LVC), five of them associated with black hole binary (BHB) mergers and one with a double neutron star (DNS) merger, while the coalescence of a black hole-neutron star (BHNS) binary is still missing. We investigate the progenitors of double compact object binaries with our population-synthesis code MOBSE. MOBSE includes advanced prescriptions for mass loss by stellar winds (depending on metallicity and on the Eddington ratio) and a formalism for core-collapse, electron-capture and (pulsational) pair instability supernovae. We investigate the impact of progenitor's metallicity, of the common-envelope parameter $\alpha{}$ and of the natal kicks on the properties of DNSs, BHNSs and BHBs. We find that neutron-star (NS) masses in DNSs span from 1.1 to 2.0 M$_\odot$, with a preference for light NSs, while NSs in merging BHNSs have mostly large masses ($1.3-2.0$ M$_\odot$). BHs in merging BHNSs are preferentially low mass ($5-15$ M$_\odot$). BH masses in merging BHBs strongly depend on the progenitor's metallicity and span from $\sim{}5$ to $\sim{}45$ M$_\odot$. The local merger rate density of both BHNSs and BHBs derived from our simulations is consistent with the values reported by the LVC in all our simulations. In contrast, the local merger rate density of DNSs matches the value inferred from the LVC only if low natal kicks are assumed. This result adds another piece to the intricate puzzle of natal kicks and DNS formation.

Formation of Double Neutron Stars, Millisecond Pulsars and Double Black Holes

The 1982 model for the formation of the Hulse-Taylor binary radio pulsar PSR B1913+16 is described, which since has become the standard model for the formation of double neutron stars, confirmed by the 2003 discovery of the double pulsar system PSR J0737-3039AB. A brief overview is given of the present status of our knowledge of the double neutron stars, of which 15 systems are presently known. The binary-recycling model for the formation of millisecond pulsars is described, as put forward independently by Alpar et al. (1982), Radhakrishnan and Srinivasan (1982) and Fabian et al. (1983). This now is the standard model for the formation of these objects, confirmed by the discovery in 1998 of the accreting millisecond X-ray pulsars. It is noticed that the formation process of close double black holes has analogies to that of close double neutron stars, extended to binaries of larger iinitial component masses, although there are also considerable differences in the physics of the binary evolution at these larger masses.

TWIN BINARIES: STUDIES OF STABILITY, MASS TRANSFER, AND COALESCENCE

Motivated by suggestions that binaries with almost equal-mass components ("twins") play an important role in the formation of double neutron stars and may be rather abundant among binaries, we study the stability of synchronized close and contact binaries with identical components in circular orbits. In particular, we investigate the dependency of the innermost stable circular orbit on the core mass, and we study the coalescence of the binary that occurs at smaller separations. For twin binaries composed of convective main-sequence stars, subgiants, or giants with low mass cores (M_c <~0.15M, where M is the mass of a component), a secular instability is reached during the contact phase, accompanied by a dynamical mass transfer instability at the same or at a slightly smaller orbital separation. Binaries that come inside this instability limit transfer mass gradually from one component to the other and then coalesce quickly as mass is lost through the outer Lagrangian points. For twin giant binaries with moderate to massive cores (M_c >~0.15M), we find that stable contact configurations exist at all separations down to the Roche limit, when mass shedding through the outer Lagrangian points triggers a coalescence of the envelopes and leaves the cores orbiting in a central tight binary. In addition to the formation of binary neutron stars, we also discuss the implications of our results for the production of planetary nebulae with double degenerate central binaries.

The long and the short of it: modelling double neutron star and collapsar Galactic dynamics

The work presented here examines populations of double compact binary systems and tidally enhanced collapsars. We make use of BINPOP and BINKIN, two components of a recently developed population synthesis package. Results focus on correlations of both binary and spatial evolutionary population characteristics. Pulsar and long duration gamma-ray burst observations are used in concert with our models to draw the conclusions that: double neutron star binaries can merge rapidly on timescales of a few million years (much less than that found for the observed double neutron star population), common envelope evolution within these models is a very important phase in double neutron star formation, and observations of long gamma-ray burst projected distances are more centrally concentrated than our simulated coalescing double neutron star and collapsar Galactic populations. Better agreement is found with dwarf galaxy models although the outcome is strongly linked to the assumed birth radial distribution. The birth rate of the double neutron star population in our models range from 4-160 Myr^-1 and the merger rate ranges from 3-150 Myr^-1. The upper and lower limits of the rates results from including electron capture supernova kicks to neutron stars and decreasing the common envelope efficiency respectively. Our double black hole merger rates suggest that black holes should receive an asymmetric kick at birth.

The Lowest-Mass Stellar Black Holes: Catastrophic Death of Neutron Stars in Gamma-Ray Bursts

Mergers of double neutron stars are considered the most likely progenitors for short gamma-ray bursts. Indeed, such a merger can produce a black hole with a transient accreting torus of nuclear matter, and the conversion of a fraction of the torus mass-energy to radiation can power a gamma-ray burst. Using available binary pulsar observations supported by our extensive evolutionary calculations of double neutron star formation, we demonstrate that the fraction of mergers that can form a black hole-torus system depends very sensitively on the (largely unknown) maximum neutron star mass. We show that the available observations and models put a very stringent constraint on this maximum mass under the assumption that black hole formation is required to produce a short gamma-ray burst in a double neutron star merger. Specifically, we find that the maximum neutron star mass must be within 2–2.5 M☉. Moreover, a single unambiguous measurement of a neutron star mass above 2.5 M☉ would exclude a black hole-torus central engine model of short gamma-ray bursts in double neutron star mergers. Such an observation would also indicate that if in fact short gamma-ray bursts are connected to neutron star mergers, the gamma-ray burst engine is best explained by the lesser known model invoking a highly magnetized massive neutron star.

The Role of Helium Stars in the Formation of Double Neutron Stars

We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of MHe = 2.5-6 M☉ with a 1.4 M☉ neutron star companion to investigate the formation of double neutron star systems. Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P ≲ 0.1 days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with MHe. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (~2.6-3.3 M☉). The existence of a short-period population of double neutron stars increases the predicted detection rate of in-spiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as γ-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass transfer results for the orbital properties of DNS populations.

Merger Sites of Double Neutron Stars and Their Host Galaxies

Using the StarTrack population synthesis code, we analyze the formation channels possibly available to double neutron star binaries and find that they can be richer than previously thought. We identify a group of short-lived, tight binaries, which do not live long enough to escape their host galaxies, despite their large center-of-mass velocities. We present our most recent results on all possible evolutionary paths leading to the formation of double neutron stars, calculate their coalescence rates, and also revisit the question of the distribution of merger sites around host galaxies. For a wide variety of binary evolution models and galaxy potentials, we find that most of the neutron star mergers take place within galaxies. Our results stem from allowing for radial and common envelope evolution of helium-rich stars (testable in the future with detailed stellar-structure and hydrodynamic calculations) and indicate that double neutron star binaries may not be excluded as gamma-ray burst progenitors solely on the basis of their spatial distribution around host galaxies. We also find, in contrast to Bethe & Brown, that in a significant fraction of common envelope (CE) phases, neutron stars do not accrete enough material to become black holes, and thus the channels involving CEs are still open for the formation of double neutron stars.